1. Field of the Invention
The present invention relates to a particle beam irradiation system, and more particularly to a particle beam irradiation system that is suitably applicable to a particle beam therapy system, which provides cancer treatment by irradiating a diseased part with a proton, heavy ion, or other charged particle beam.
2. Description of the Related Art
Particle beam therapy is known as radiological therapy of cancer. In particle beam therapy, a patient's cancerous part is irradiated with a proton, heavy ion, or other ion beam for therapy purposes. A particle beam therapy system that provides particle beam therapy includes an ion beam generator, a beam transport line, and an irradiation apparatus. The ion beam generator includes a synchrotron or cyclotron that accelerates an ion beam, which moves in a circular path, to a desired energy.
The synchrotron includes a radiofrequency acceleration system (radiofrequency acceleration cavity), an extraction radiofrequency electrode, and an extraction deflector (refer, for instance, to Patent Document 1). The radiofrequency acceleration system applies a radiofrequency voltage to the ion beam, which moves in a circular path, to accelerate the ion beam to a target energy. The extraction radiofrequency electrode increases the betatron oscillation amplitude of the ion beam, which moves in a circular path. The extraction deflector removes the ion beam from its circular path. When the ion beam accelerated to the target energy is to be extracted from the synchrotron to the beam transport line, the extraction radiofrequency electrode applies a radiofrequency magnetic field or radiofrequency electric field (hereinafter referred to as the radiofrequency signal) to the ion beam to increase the betatron oscillation amplitude, which represents the intrinsic oscillation of the ion beam that moves in a circular path. After the betatron oscillation amplitude is increased, the ion beam moves beyond a stability limit, is extracted from the synchrotron to the beam transport line, and is transported to the irradiation apparatus.
The cyclotron receives the ion beam, which is supplied from an ion source, applies a radiofrequency voltage to the ion beam in a uniform magnetic field within the cyclotron, and causes the ion beam to move out of the circular path. After being accelerated to the target energy, the ion beam is extracted to the beam transport line and transported to the irradiation apparatus.
The irradiation apparatus shapes the ion beam, which is introduced from the ion beam generator, in accordance with the depth from the patient body surface and the shape of a diseased part, and irradiates the diseased part of a patient in a therapy bed with the shaped ion beam. The irradiation apparatus irradiates the diseased part with the ion beam by using an appropriate beam irradiation method. In general, the irradiation apparatus uses a double scatterer method (Non-patent Document 1, page 2081, FIG. 35, wobbler method (Non-patent Document 1, page 2084, FIG. 41), or beam scanning method (Patent Document 1 or Non-patent Document 1, pages 2092 and 2093).
The diseased part usually has a certain thickness in the direction of ion beam propagation in a patient body. To irradiate the whole thickness of the diseased part with the ion beam, it is necessary to control the energy of the ion beam so as to form a uniform absorbed dose range (spread-out Bragg peak or SOBP) having a certain width in the direction of ion beam propagation. An irradiation method that uses a range modulation wheel (hereinafter referred to as the RMW) is proposed as energy control means for forming a desired SOBP. The RMW is a rotating structure and has a plurality of wedge-shaped energy absorbers arranged in circumferential direction so that the thickness of a region through which the ion beam passes varies with time. When the RMW rotates, the thickness in the direction of ion beam propagation (the axial direction of the RMW) increases or decreases. An RMW-based irradiation method (Non-patent Document 1, page 2077, FIG. 30) is called an RMW irradiation method.
The irradiation apparatus shapes the ion beam in accordance with the depth from the patient body surface and the shape of a diseased part. However, the intensity of the beam incident on the diseased part is adjusted by the ion beam generator. In the synchrotron, the beam intensity of the ion beam that is extracted from the ion beam generator is controlled as desired by adjusting the intensity of the radiofrequency signal to be applied to the extraction radiofrequency electrode. The cyclotron includes a device that adjusts the intensity of an extracted ion beam (refer, for instance, to Patent Document 2). More specifically, the cyclotron measures the intensity of an actually extracted beam, and exercises control in accordance with the measurement result to regulate an arc current that is to be supplied to the ion source.    Patent Document 1: Japanese Patent No. 2596292    Patent Document 2: JP-A-2004-529483 (PCT)    Non-patent Document 1: Review of Scientific Instruments, Volume 64, Number 8 (August 1993), pages 2074-2093    Non-patent Document 2: Proceedings of the Symposium on Accelerator and Related Technology for Application, Volume 7 (June 2005), pages 35-36